The present invention relates to an ultrasonic motor which employs a longitudinal-torsional composite vibrator consisting of a stator as a generation source of a rotational torque, and rotates a rotor pressed on the stator via a frictional force.
The ultrasonic motor utilizes a rotational torque received through a frictional force by a rotor pressed against a stator which serves as a vibrator for causing ultrasonic elliptical vibration.
Demand has arisen for a motor which has a small diameter and a large torque. As an ultrasonic motor having such a function, the present inventors proposed an ultrasonic motor which uses a longitudinal torsional composite vibrator as a stator, disclosed in Proc. 1988 Autumn meeting, Acoustic Society of Japan No. 2-4-10, pp. 821 to 822 (Oct., 1988).
FIG. 13 shows the arrangement of this prior art ultrasonic motor. In FIG. 13, reference numeral 11 denotes a piezoelectric ceramic element for causing longitudinal vibration. The element 11 is subjected to a polarization treatment in a direction of thickness. Reference numeral 12 denotes a piezoelectric ceramic element for causing torsional vibration. The element 12 is subjected to a polarization treatment in a circumferential direction parallel to its surface. These piezoelectric elements 11 and 12 are securely fastened by a head mass 13, a rear mass 14, and a bolt 15, which are formed of an Al alloy, thereby constituting a stator 30 as an ultrasonic elliptical vibrator. Reference numeral 20 denotes a spring for biasing a rotor 17 against the stator; 19, a base; 22, a shaft; and 21, a nut. The biasing force of the spring can be adjusted by the nut 21. FIG. 14 shows the operational principle of the ultrasonic motor. The longitudinal vibration serves as "clutch", and a torsional displacement in only one direction is transmitted to the rotor.
The ultrasonic motor has been proposed to aim at resonantly driving longitudinal and torsional vibrations at the same time in order to efficiently and strongly excite elliptical vibration obtained by synthesizing the longitudinal and torsional vibrations at the interface between the stator and rotor. In order to perform resonant driving, the resonant frequencies of the longitudinal and torsional vibrations must coincide with each other. In the ultrasonic motor shown in FIG. 13, a shaft having a proper thickness stands upright on the stator to adjust a pressing force between the rotor and the stator, so that the resonant frequencies of the longitudinal and torsional vibrations can marginally coincide with each other in a weak electric field mode.
The resonant frequencies of the longitudinal and torsional vibrations can coincide with each other in the weak electric field mode. However, when a motor is driven in a strong electric field mode in practice, a resonant frequency f.sub.T of the torsional vibration becomes higher than a resonant frequency f.sub.L of the longitudinal vibration. Thus, it is difficult to cause resonant frequencies to coincide with each other in the strong electric field mode as an actual driving state. In the ultrasonic motor having the arrangement shown in FIG. 13, the resonant frequency f.sub.T of the torsional vibration is almost determined by the length of the stator portion, and is not so influenced by the pressing force. However, the resonant frequency f.sub.L of the longitudinal vibration depends on the mass of the rotor and the pressing force between the rotor and the stator. As the mass of the rotor is smaller and the pressing force is larger, the resonant frequency f.sub.L becomes closer to that of the torsional vibration.
More specifically, in the ultrasonic motor having the arrangement shown in FIG. 13, f.sub.T &gt;f.sub.L in general. Therefore, in order to achieve f.sub.T =f.sub.L, the weight of the rotor must be reduced. For this purpose, the height of the rotor must be decreased. However, the rotor having such a shape provides only a small rigidity, and can hardly generate a large torque. Alternatively, the pressing force must be extremely increased. An extreme increase in pressing force inevitably applies an excessive stress to a bearing, and leads to damage to and a short service life of the bearing. Therefore, in the conventional ultrasonic motor shown in FIG. 13, f.sub.T is higher than f.sub.L in an actual high-power driving state, and total efficiency is at most about 25% to 40%.